Vectors: Tsetse flies

Previous authors: R J PHELPS AND D F LOVEMORE

Current authors:
J BOUYER - Medical Entomologist, DVM, PhD, HDR, Insect Pest Control Laboratory, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Vienna, Austria & UMR CIRAD-Inra ASTRE « Animal, Santé, Territoires, Risques et Ecosystèmes », Montpellier, France
M J B VREYSEN - Laboratory Head, MSc, PhD, Insect Pest Control Laboratory, Wagramerstrasse 5, Vienna, Austria

Introduction

Blood-sucking muscoid Diptera of the family Glossinidae and the genus Glossina, known as tsetse flies, are restricted almost entirely to sub-Saharan Africa and some of its off-shore islands such as Mafia Island, although they are absent from others such as Madagascar and Pemba Island of Zanzibar. There are old records of tsetse flies from south-west Arabia,⁶⁴ and near Gizar in Saudi Arabia,¹¹⁷ but they have not been reported there since then. It had long been known that domestic stock could not be kept where tsetse flies occurred and that disease was associated with the flies. However, it was not until Bruce, working in KwaZulu-Natal, South Africa in 1895, demonstrated the transmission of pathogenic trypanosomes of livestock by tsetse flies that the reason for livestock deaths became known. The role of tsetse flies as vectors of human trypanosomosis, and the importance of the Glossina palpalis group in this respect, was also demonstrated by Bruce and his co-workers in 1909.⁵⁰ Nash (1969)²⁷⁰ appropriately referred to tsetse flies as ‘Africa’s bane’²⁷⁰ as they are the sole cyclical vectors of trypanosomes, the causative agents of ‘sleeping sickness’ or human African trypanosomosis (HAT) in humans and ‘nagana’ or African animal trypanosomosis (AAT) in livestock.  During feeding on an infected host, the parasites are together with the blood, ingested by the fly, after which they undergo a cyclical development cycle in the fly.

Although there are 31 species and subspecies of tsetse flies²⁵⁷ all placed in the genus Glossina of the family Glossinidae,⁶⁰ only eight to 10 species are considered of economic (agricultural-veterinary) or human sanitary importance.  Tsetse flies occur in 38 African countries and infest an area of close to 10 million km². These areas could not be explored easily before the advent of mechanized transport, as animal-drawn transport was excluded by AAT. The presence of tsetse flies was thus a major obstacle to the development of much of the continent, a situation that obtains in many places even today.⁸

There are those, even now, who look upon the tsetse fly as the guardian of the natural ecosystems of Africa, and who would like this fly to remain until humans have learnt to manage the land in a sustainable manner. At present, though, the human population of Africa is expanding rapidly, and the economic situation is such that the burgeoning population can, for the most part, be accommodated only as peasant farmers. The importance of livestock to such people is great, not only as a source of food, draught power, and money, but also for the important role that livestock, especially cattle, play in cultural affairs. Under these circumstances it is essential to try to manage the diseases of livestock, amongst which AAT plays a major role.

In view of the vast areas infested with tsetse flies, it is estimated that 60 million people are constantly exposed to the risk of becoming infected with HAT. Whereas in 2001, according to the World Health Organization, between 300 000-500 000 people were still suffering from HAT.⁶⁵ The prevalence of HAT has declined drastically in the last decade due to increased surveillance, readily available drugs, more trained technicians, and a greater commitment of the international community.^{323, 324}

Socio-economic impact

Tsetse flies, hunger, poverty

Hunger is the most extreme manifestation of poverty and with 34 per cent of the population being undernourished, it remains acute in rural sub-Saharan Africa. Alleviation of poverty can only start with the reduction of hunger and this can be achieved through the development of sustainable agricultural systems. Livestock is an essential part of this as it provide milk and meat for nourishment, transport, hides, manure for fuel or fertilizer, and it can support crop production through the provision of draught power. It is also an important means of savings for the farmers and is an essential source of income for the rural poor.¹²⁴ Productive livestock is, however, largely absent in these vast fertile tsetse infested areas due to the omnipresence of an inconspicuous insect: the tsetse fly.³⁹⁶

The socio-economic impact of tsetse flies in sub-Saharan Africa is enormous. Direct annual cattle production losses due to the presence of the tsetse fly and AAT have been estimated at USD 600–1200 million,¹⁷⁴ and the overall annual lost potential in livestock and crop production has been estimated at USD 4750 million.⁵¹ Importantly, the tsetse fly prevents the integration of crop farming and livestock keeping, which is considered crucial to the development of more efficient and sustainable livestock production systems.⁸ The presence of the tsetse fly and AAT is therefore considered to be one of the major root causes of hunger and poverty in sub-Sahara Africa; hence, the tsetse fly has been appropriately termed “the poverty fly”.  This is evidenced by the remarkable overlap between the 38 tsetse-infested countries and the 34...